Adventures in Thermochemistry. James S. Chickos * Department of Chemistry and Biochemistry University of Missouri-St. Louis Louis MO 63121 E-mail: [email protected] 10. Confluence of the Missouri and Mississippi Rivers. Applications of the The Correlation-Gas Chromatographic Method
Objectives: To go where no one else has gone
1. Evaluation of the vaporization enthalpies of large molecules
2. Application of Correlation-Gas Chromatography to a Tautomeric Mixture - Acetylacetone
Dialkyl phthalates and related isomers are important industrial products, and many have been produced in large quantities for a considerable period of time. Their importance ranges from their use in polymers as plasticizers to applications in cosmetics.
Some plastics become brittle with time simply due to the evaporation of the plasticizer
Phthalate esters have been in use for a considerable amount of time. Due to the
nature of their properties, and longevity of use, they are ubiquitous in the environment. The vapor pressures and vaporization measured repeatedly over the years. This has led to large discrepancies in their thermodynamic properties.
Dibutyl phthalate and bis (2-ethylhexyl) phthalate have been selected as reference compounds for vapor pressure measurements by the US EPA.
We decided to examine if it were possible to establish a set of self consistent experimental values in an area that has been characterized by numerous discordant values. We are not aware of any other method capable of this.
As an example of the problems associated with these compounds, we purchased diocyl phthalate and terephthalate from Aldrich only to discover that both materials were actually the 2-ethylhexyl derivatives. Di-n-octyl phthalate is also referred to as dioctyl phthalate.
lgH(298 K)/ kJ·mol-1 = 4.69(nC - nQ) + 1.3·nQ + ΣFi·bi + 3.0 + C where the value if Fi depends on the hybridization and substitution pattern of the carbon to which the functional group is attached.
b (ester) = 10.5 kJ·mol-1; C(H3)- : F = 1.62, -C(C)(H2)-, F = 1.08, =C(C3)-, F = 0.85,
C(C2)(H)- = 0.6; C = -2 kJ·mol-1/C branch on an sp3 hybridized carbon; average deviation ~ ± 8%
ln( groupsp/po) = A’ – B’/RT (1)
No references to the original work are provided. Therefore we did not use the vapor pressure values as standards
Vapor pressures reported as: groups
ln (p/po) = (1-To/T)exp[Ao +A1(T/K) +A2(T/K)2] Cox Eq
Vapor pressures reported as: ln( groupsp/po) = a + b(T/K)-1 + c(T/K)-2
(f) Small, P. A.; Small, K. W.; Cowley, P. The Vapor Pressure of Some High Boiling Esters. Trans. Faraday Soc. 1948, 44, 810-6.
lgH( 298.15 K) =
(95.0 1.1) kJmol-1
Average value from Hales et al.; Verevkin, and this work
Figure. Bottom curve: A plot of ln( groupsp/po) versus T/K for vapor pressures reported for dibutyl phthalate by Small et al., (line),17 Hales et al. ( ),18 and this work, transpiration (▲).
This process was repeated at groupsT = 10 K intervals from (298.15 to 550) K and the
resulting vapor pressures fit to the following third order polynomial (r 2 > 0.99)
ln(p/po) = A”(T/K)-3 + B”(T/K)-2 + C”(T/K)-1 + D” (9)
A duplicate run resulted in the same value for groupsbis (2-ethylhexyl) phthalate. Since this value is within experimental error of the EPA value, the EPA value was used of 116.7±0.5 kJmol-1was used in subsequent runs
bis 2-ethylhexyl phthalate116.7 groups±0.5 EPA value
Using vapor pressures for: groups
vapor pressures were evaluated from correlations between ln(to/ta) and ln(p/po) as a function of temperature from T = 298.15 to 550 K
Dimethyl terephthalate and isophthalate, and dicyclohexyl phthalate are crystalline solids at room temperature. Since vapor pressures of the liquid as a function of temperature, vaporization and fusion enthalpies at and T = (Tfus and 298.15 K) are available, it is also possivle to calculate both sublimation vapor pressures and enthalpies.
ln(pcr/Pa) = [crgH (Tfus) + crgCp·T] [ 1/Tfus/K – 1/298.15]/R + ln(p(Tfus/Pa))
where crgCp·T = [0.75 + 0.15·Cp(cr)][(Tfus /K -298.15)/2]
Visitng Undergraduate Student phthalate are crystalline solids at room temperature. Since vapor pressures of the liquid as a function of temperature, vaporization and fusion enthalpies at and
Mikhail Kozlovskiy from Moscow
Massiel Mori from South America
High School Student
Sergey Verevkin, University
of Rostock, Rostock Germany